Generally, “zeolite” has been long a generic term of crystalline porous aluminosilicates and these are (SiO4)4− and (AlO4)5− having a tetrahedral structures as the basic units of the structure. However, in recent years, it has been clarified that a structure peculiar or analogous to zeolite is present in many other oxides such as aluminophosphate.
International Zeolite Association (hereinafter simply referred to as “IZA”) organizes the frameworks of zeolite and zeolite-like materials in Atlas of Zeolite Structure Types, 5th edition, edited by Ch. Baerlocher, W. M. Meier and D. H. Olson, Elsevier, 2001 (Non-Patent Document 1) (hereinafter simply referred to as “Atlas”) and each framework is denoted by an IZA code composed of three alphabetical letters.
With respect to the details of the history thereof, “Zeolite no Kagaku to Kogaku (Science and Engineering of Zeolite” by Yoshio Ono and Tateaki Yajima (compilers), Kodansha K. K., published on Jul. 10, 2000 (Non-Patent Document 2) may be referred to.
The definition of “zeolite” as used in the present invention is based on the definition described in Zeolite no Kagaku to Kogaku (Science and Engineering of Zeolite) that zeolite includes not only aluminosilicate but also those having an analogous structure, such as metallosilicate.
In the present invention, a structure code composed of three alphabetical capital letters derived from the names of standard substances initially used for the clarification of structure, approved by IZA, is used for the structure of zeolite. This includes those recorded in Atlas and those approved in the 5th and later editions.
Further, unless otherwise indicated specifically, the “aluminosilicate” and “metallosilicate” as used in the present invention are not limited at all on the difference such as crystalline/non-crystalline or porous/non-porous and include “aluminosilicates” and “metallosilicates” in all properties.
The “molecular sieve” as used in the present invention is a substance having an activity, operation or function of sieving molecules by the size and includes zeolite. This is described in detail in “Molecular Sieve” of Hyojun Kagaku Yogo Jiten (Glossary for Standard Chemistry), compiled by Nippon Kagaku Kai, published by Maruzen on Mar. 30, 1991 (Non-Patent Document 3).
Zeolite and zeolite-like materials have various frameworks and the framework approved by IZA includes 133 species until the issue of Atlas, 5th edition. Even at present, new frameworks are being discovered and the frameworks approved by IZA are introduced on the homepage thereof.
However, the frameworks reported all are not always useful in industry and industrially useful frameworks are relatively limited. It is considered that the industrial value is generally determined by the uniqueness of structure, the production cost and the like. Among frameworks discovered in recent years, MWW structure is particularly useful in industry and attracting an attention. The MWW structure is a framework peculiar to zeolite represented by MCM-22.
According to Zeolite no Kagaku to Koqyo (Science and Engineering of Zeolite), a patent application for a synthesis method of MCM-22 was filed by Mobil in 1990 (JP-A (unexamined published Japanese patent application) 63-297210 (Patent Document 1)) and thereafter, Leonowicz et al. reported that MCM-22 is a hexagonal zeolite having a particular pore structure. A representative substance thereof is borosilicate having the following unit cell composition:H2.4Na3.1[Al0.4B5.1Si66.5O144]
The characteristic feature in the framework is to have two pore networks independent of each other in the direction perpendicular to the c axis (in the plane direction of layer). One of these pore networks is present between layers and a cocoon-like supercage (0.71×0.71×1.82 nm) is two-dimensionally connected to six supercages therearound. The supercages are directly connected to each other by a 10-membered ring and therefore, a relatively large molecule can enter into the pore as compared with a tunnel-like 10-membered ring pore. Another of the above pore networks is present within a layer and a two-dimensional network is formed by 10-membered ring zigzagged pores. ITQ-1 which is pure silica, SSZ-25 and the like have the same framework.
As for the production process for MWW-type zeolite, there is a process utilizing a hydrothermal synthesis at around 150° C. using a relatively inexpensive hexamethyleneimine as the crystallizing agent. Aluminosilicate can be synthesized at an Si/Al molar ratio of 15 to 35. Substances obtained by the hydrothermal synthesis and showing a production behavior different from zeolite are generally a layered precursor (commonly called MCM-22(P)) and are characterized in that when calcined, dehydration condensation takes place between layers and MCM-22 having a zeolitic 3-dimensional structure is formed.
However, in recent studies, it has been reported that MCM-49 produced by the same preparation method while charging a large amount of an alkali metal has the same framework as MCM-22. This reveals that not a layered precursor but aluminosilicate having an MWW structure can be directly obtained as a product of the hydrothermal synthesis (see, S. L. Lawton et al., J. Phys. Chem., 100, 3788 (1996) (Non-Patent Document 4)).
The MWW structure has a characteristic feature not seen in conventional zeolites as described above, and aluminosilicate having the MWW structure is known to exhibit high activity and selectivity in the synthesis of ethylbenzene or cumene as compared with zeolite having other structures or catalysts other than zeolite and it is considered that such zeolites have already been used in many plants over the world.
Also, there is an attempt to obtain a catalyst having higher performance by utilizing the layered precursor obtained in the synthesis of MWW structure. More specifically, MCM-36 obtained by crosslinking the layered precursor with silica (see, for example, W. J. Roth et al., Stud. Surf. Sci. Catal., 94, 301 (1995) (Non-Patent Document 5)), thin-layered substance ITQ-2 obtained by exfoliation of layers (see, for example, A. Corma et al., Microporous Mesoporous Mater., 38, 301 (2000) (Non-Patent Document 6)) and the like have been reported and it is stated that these exhibit higher activity than aluminosilicate having a mere MWW structure.
However, even in the above-mentioned high-performance catalysts, the reactivity thereof is basically derived from the layered structure constituting the MWW structure and when compared with zeolites having other frameworks, these are classified into substances analogous to zeolite having an MWW structure. The synthesis of such a zeolite-like layered compound is characterized by having a step of treating the layered precursor MCM-22(P) in an aqueous solution containing a surfactant such as hexadecyltrimethylammonium bromide, and thereby swelling or exfoliating a layer.
On the other hand, since the MWW structure has a characteristic feature not seen in other zeolite structures as described above, a characteristic catalytic activity or adsorbing activity attributable to the MWW framework structure can be expected. This characteristic activity is not necessarily limited to the above-described aluminosilicate but metallosilicate containing an element other than aluminum in the framework (or skeleton) is also expected to provide the same effect. From this expectation, various studies have been made on the synthesis of metallosilicate having an MWW structure. However, the transition element represented by titanium, vanadium and chromium, and the typical element of the 5th period or more represented by indium and tin, which are expected to show remarkably different properties from aluminosilicate in general (not limited to MWW structure), have a very large ionic radius as compared with silicon or aluminum and therefore, such an element is usually difficult to introduce into the framework. By the simple direct synthesis method of allowing a compound containing such an element to be present together in the raw material for synthesizing zeolite, a desired metallosilicate cannot be obtained in many cases.
For introducing the element into the framework, various methods have been proposed. Representative examples of the method employed for the MWW structure include a post-synthesis method (a method of once synthesizing zeolite and after-treating it to introduce a heteroelement into the framework; this is generally called a post-synthesis in contract with the direct synthesis) and an improved direct method.
With respect to the post-synthesis method, for example, U.S. Pat. No. 6,114,551 (Patent Document 2) discloses a process for synthesizing metallosilicate by a post-synthesis method, where aluminosilicate having an MWW structure is once synthesized, the whole or a part of aluminum is removed out from the aluminosilicate by a dealuminating treatment such as contact with SiCl4 in gas phase to form defects, and a compound containing an element to be introduced, such as TiCl4, is contacted with the dealuminated product.
As for the improved direct method, Wu et al. have reported an example where ferrisilicate is obtained by designing the step of adding an iron compound to a gel (P. Wu et al., Chem. Commun., 663 (1997) (Non-Patent Document 7)).
Further, for Ti which is difficult to introduce into the frame, a synthesis method using boron as a structure supporting agent has been recently developed (P. Wu et al., Chemistry Letters, 774 (2000) (Non-Patent Document 8)).
Also, a method for obtaining MWW-type titanosilicate has been proposed, where a large amount of boron is added to a starting raw material, an MWW precursor MCM-22(P) having both boron and titanium in the framework is synthesized by utilizing the function of boron as a structure supporting agent and after removing boron, if desired, by an acid treatment, the obtained precursor is calcined. The titanosilicate having an MWW structure prepared by this method is reported to exert a characteristic catalytic activity (P. Wu et al., J. Phys. Chem. B, 105, 2897 (2001) (Non-Patent Document 9)).
However, according to these methods, particularly the post-synthesis method wherein a zeolite is caused to contact a compound of an element to be introduced thereinto, most part of the elements intended to introduce cannot be introduced into the framework and remain as a residue in the pore. In order to improve the introduction efficiency, one important point is to select a compound which can easily enter into pores of zeolite. However, there is a problem that in general a compound containing an element intended to introduce and having a sufficiently small molecular size is not available on the market.
Further, on use as a catalyst or the like, in the case where the raw material is a dealuminated product of MWW-type aluminosilicate as in U.S. Pat. No. 6,114,551, a side reaction ascribable to aluminum remaining in the framework sometimes brings about a serious problem such as causing side reactions to provide by-products. The same problem occurs in the direct method using boron as a structure supporting agent, that is, boron cannot be satisfactorily removed even by an acid treatment and a large amount of boron remains in the framework or pores, or if strict conditions are adapted for the process of removing boron by an acid treatment or the like so as to enhance the removal ratio of boron, components which must remain in the frame are also disadvantageously removed at the same time. Moreover, the proper synthesis conditions greatly change depending on the element intended to introduce and the compound containing the element and therefore, these methods are not very good in terms of the general-purpose applicability.
(Patent Document 1)
JP-A 63-297210
(Patent Document 2)
U.S. Pat. No. 6,114,551
(Non-Patent Document 1)
Atlas of Zeolite Structure Types, 5th edition, edited by Ch. Baerlocher, W. M. Meier and D. H. Olson, Elsevier, 2001
(Non-Patent Document 2)
“Zeolite no Kagaku to Kogaku (Science and Engineering of Zeolite” by Yoshio Ono and Tateaki Yajima (compilers), Kodansha K. K., published on Jul. 10, 2000
(Non-Patent Document 3)
“Molecular Sieve” of Hyojun Kagaku Yogo Jiten (Glossary for Standard Chemistry), compiled by Nippon Kagaku Kai, published by Maruzen on Mar. 30, 1991.
(Non-Patent Document 4)
S. L. Lawton et al., J. Phys. Chem., 100, 3788 (1996)
(Non-Patent Document 5)
W. J. Roth et al., Stud. Surf. Sci. Catal., 94, 301 (1995)
(Non-Patent Document 6)
A. Corma et al., Microporous Mesoporous Mater., 38, 301 (2000)
(Non-Patent Document 7)
P. Wu et al., Chem. Commun., 663 (1997)
(Non-Patent Document 8)
P. Wu et al., Chemistry Letters, 774 (2000)
(Non-Patent Document 9)
P. Wu et al., J. Phys. Chem. B, 105, 2897 (2001)